When a useful gene product is produced in a genetically engineering manner, depending upon its purpose, there has been used an expression system using as a host a microbial cell such as Escherichia coli, Bacillus subtilus, or an yeast; an animal cell, an insect cell, a plant cell or the like of which cultivation means has been established, and using a promoter appropriate for the host. Among them, an expression system using Escherichia coli as a host and lac promoter or a derivative thereof or the like is a kind of system which is often used from the viewpoint of its facility in handling.
However, the expression system using lac promoter or a derivative thereof has some defects such that the expression system is industrially disadvantageous because the induction of gene expression is necessitated during expression of the gene product. For instance, lac promoter, tac promoter or the like has some defects such that it is disadvantageous to carry out the above expression on an industrial scale, because expensive IPTG (isopropyl-β-D-thiogalactopyranoside) is required for the induction of gene expression.
On the other hand, an expression vector utilizing temperature induction of phage λ promoter has also been generally used. However, in temperature-induced overexpression of a recombinant gene product, there may be some disadvantages of (a) difficulty for achieving a rapid shift-up of temperature; (b) increase in possibility of the formation of insoluble inclusion bodies at a higher culturing temperature; and (c) induction of some proteases in Escherichia coli during heat shock.
Therefore, there has been desired a means which allows highly efficient expression without inducing gene expression.